The invention relates to an apparatus for determining light reflection from a focal plane shutter, and more particularly, to such apparatus which determines a proper exposure period automatically by the photometry of light from an object being photographed which is reflected by a shutter blind surface or a film surface or both.
A photometric device used in a camera has for its object the measurement of light from an object being photographed in order to assure a proper exposure of a film. Accordingly, a photometric, photoelectric transducer element is ideally disposed on a film surface or an equivalent surface such as a shutter blind of a focal plane shutter. However, such disposition of the element is impossible in practice, but a so-called direct photometry technique is conventionally employed in which a measurement of light from an object being photographed is made for which purpose the light is reflected by a film surface or an equivalent shutter blind surface. As is well recognized, when a focal plane shutter is used, the film surface is initially covered by the first blind of the shutter which is formed by a black cloth. As the first blind moves across the picture frame, starting in response to a shutter release, the film surface which has been covered by the first blind is increasingly exposed. When a proper exposure period has passed, the second blind of the shutter which is also formed by a black cloth starts to move across the screen to cover the exposed film surface.
With a high speed shutter operation or when the exposure period is very short, the second blind is caused to start running before the first blind has terminated its running so that a reduced exposure period may be obtained. As the shutter period is reduced, the width of the slit defined between the first and the second blind decreases in order to determine a proper exposure period. A measurement is made of light from an object being photographed, which light is initially reflected by the surface of the first blind of the shutter and then by the film surface which is being exposed. Because the shutter blind and the film surface have different values of optical reflectivity, some correction must be made to provide uniform optical reflectivity since otherwise the measurement of the reflected light cannot be correctly made, resulting in a failure to provide a proper exposure.
Accordingly, in prior art photometric devices of this type, the surface of the first blind is printed with a pattern of a material which exhibits the same reflectivity as the film surface. A shutter blind is usually formed by a front side cloth having a rubber lining on the rear side. Hence, it is very difficult to provide a printing on the front surface, resulting in a very expensive construction. In addition, variation in the pattern being printed causes a change in the reflectivity. It will also be noted that since the shutter blind is taken up into a roll at high speed, the blind surface has a poor planarity. The patterned coating or painting may be exfoliated. Finally, it is recognized that a black, delustering paint is applied to the interior of a camera in order to reduce stray light within a mirror box and leakage onto the film or to prevent a ghost or a flaring. However, when a reflecting pattern is printed on the surface of the first blind in order to achieve the same reflectivity as the film surface, it may interfere with the described extinction effect, causing the occurrence of a flaring or ghost.
When the surface of the first blind of the shutter is treated to provide the same reflectivity as the film surface, a photometric circuit such as shown in FIG. 1 may be used to determine the reflection. Specifically, a photoelectric transducer element D such as a silicon photodiode is used to measure light reflected from the film surface and the surface of the first blind. The reflected light is converted into a photocurrent, which then charges a capacitor C.sub.1 to provide an integrated voltage, which is then applied to one input terminal IN of a control circuit CT. A reference voltage from a source E.sub.RF is applied to the other input terminal RF of the control circuit CT, whereby the integrated voltage is compared to the reference voltage in order to determine a proper exposure period. A proposal has already been made which eliminates a disadvantage of such photometric circuit by directly using a black first shutter blind which is not provided with a printed pattern and making a correction to compensate for the differential reflectivities when determining light reflected from the blind surface and the film surface, in order to determine a proper exposure.
Referring to FIG. 2, it will be noted that the film surface which has been initially covered by the first shutter blind will be gradually exposed as the first blind moves. FIG. 2 shows a rectangular picture frame F where it will be noted that a film surface E will be gradually exposed as a first blind B moves in a direction indicated by arrow a.sub.0. The abscissa of FIG. 2 represents a time axis t which is required for movement of the first blind B. The exposure of the film surface is initiated at time T.sub.1 when the first blind B has moved past the left-hand end of the picture frame F. The picture frame F will be fully exposed at time T.sub.2 when the first blind B has moved past the right-hand end of the picture frame F. Time t=T.sub.2 represents the termination of running of the first blind, exposing the entire picture frame. FIG. 2 shows an intermediate position of the first blind B where the film surface E is partly exposed. Representing the exposed area of the film surface E by A.sub.2 and the area which is covered by the first blind B by A.sub.1, the entire area A.sub.0 of the picture frame F will be represented as A.sub.0 =A.sub.1 +A.sub.2. FIG. 3 shows a change of these areas with respect to time t. Prior to time T.sub.1 when the exposure of the film surface is initiated (t&lt;T.sub.1), the exposed area A.sub.2 of the film surface E remains zero, and the entire surface A.sub.0 of the picture frame F is covered by the first blind B, the area A.sub.1 of which is equal to the entire area A.sub.0 of the picture frame F. At time t between the time T.sub.1 and time T.sub.2 when the picture frame is fully open, the exposed area A.sub.2 of the film surface E will increase in proportion to the time t while the area A.sub.1 covered by the first blind B gradually decreases in proportion to the time t. However, the sum of the both areas A.sub.1 and A.sub.2 is equal to the entire area A.sub.0. After time T.sub.2, the area A.sub.2 of the film surface E is equal to the entire area A.sub.0 while the area A.sub.1 covered by the first blind B becomes zero.
While the respective areas vary as the first shutter blind moves relative to the film surface, a proper exposure can ideally be determined if the first blind is absent and only the light reflected by the film surface is determined. However, in practice the first shutter blind produces reflected light. Hence, the light reflected from the first blind must also be determined. However, a correction may be made to compensate for any difference between the light reflected from the blind surface and light reflected from the film surface in deriving a proper exposure period.
As mentioned previously, the determination of light reflected from the blind surface and the film surface is made by using a photoelectric transducer element such as a silicon photodiode to produce a photocurrent, which in turn charges a capacitor to provide an integrated voltage, which is then compared against a given reference voltage to determine a proper exposure period.
Referring to FIG. 4, the ordinate represents the photocurrent I.sub.P while the abscissa represents the time t. It will be apparent from the dotted line curve of this Figure that there will be a constant photocurrent I.sub.P which is equal to a photocurrent I.sub.F corresponding to the reflectivity of the film surface and which does not change with time t if the first shutter blind has the same reflectivity as the film surface. However, if the first shutter blind has a different reflectivity from that of the film surface, the photocurrent I.sub.P will vary with the movement of the first blind. In FIG. 4, times T.sub.1, T.sub.2 correspond to the same times shown in FIG. 2. Thus, at time T.sub.1, the exposure of the film surface is initiated, and at time T.sub.2, the picture frame is fully open. Time Tc represents an intermediate point between the times T.sub.1 and T.sub.2 when the first blind has passed the center of the film surface. A curve designated Is represents a photocurrent resulting from the reflection from the black surface of the first shutter blind which is not provided with a printed pattern. A curve If represents a photocurrent which results from the reflection from the film surface, as mentioned previously.
Curves I.sub.A, I.sub.AR and I.sub.B of FIG. 4 represent changes which occur in the photocurrent by the reflection from the black surface of the first shutter blind which is not provided with a printed pattern. Specifically, curve I.sub.A represents a change in the photocurrent which will be produced when the photometry is effected with a photoelectric transducer element which is oriented to receive light principally from the center of the picture frame. Until time Tc when the first shutter blind passes the center of the film surface, the photocurrent will be equal to the value Is which results from the reflection from the surface of the first blind, while after Tc, it will be equal to the photocurrent If resulting from the reflection from the film surface. It will be appreciated that the curve I.sub.A is a theoretical illustration only since although it has a vertically rise at time Tc, the element having the principal orientation or light acceptance characteristic aligned with the center of the picture frame will receive light from a region centered about the center of the film surface, whereby the actual photocurrent will be represented by the curve I.sub.AR. Thus, it has a point of deflection at time Tc, but there occurs no step change from the horizontal to the vertical, only a gradual change in the region of time Tc.
The curve I.sub.B represents a change in the photocurrent which will occur when a photoelectric transducer element having a uniform light acceptance characteristic is used for the photometry. The photocurrent increases in proportion to an increase in the exposed area of the film surface as the first blind runs. This curve I.sub.B corresponds to a change of the area A2 of the film surface shown in FIG. 3. Thus, the photocurrent represented by the curve I.sub.B is equal to the photocurrent Is resulting from the reflection from the blind surface prior to time T.sub.1, and will be equal to the photocurrent I.sub.F resulting from the reflection from the film surface after time T.sub.2.
FIG. 5 graphically shows a voltage which results from an integration of the photocurrent I.sub.P. The ordinate represents the integrated voltage Vc and the abscissa the time t. In this Figure, a rectilinear line Fv represents an ideal illustration of an integrated voltage plotted over the time where the surface of the first blind has the same reflectivity as the film surface. By contrast, curves Av, Bv represent integrated voltages corresponding to the curves I.sub.A and I.sub.B shown in FIG. 4, respectively, when the shutter blind has a black surface which is not provided with a printed pattern. Specifically, the curve Av represents the integrated voltage for the photoelectric transducer element having the principal light acceptance characteristic oriented toward the center of the picture frame while the curve Bv represents the integrated voltage for the photoelectric transducer element having the uniform light acceptance characteristic.
Time T.sub.1 on the abscissa represents the time when the exposure of the film surface is initiated while at time T.sub.2, the picture frame is fully open. At time T.sub.3, a proper exposure has been given and the second blind starts to run. Times T.sub.4a and T.sub.4b represent those times when the integrated voltage resulting from the reflection from the black surface of the first blind, not treated in any manner, becomes equal to a reference voltage V.sub.RF to be described later. T.sub.4a corresponds to the photometric transducer element having the centrally oriented principal light acceptance characteristic while T.sub.4b corresponds to the photoelectric transducer element having the uniform light acceptance characteristic. Time Tc represents the time when the first blind moves past the center of the film surface. The reference voltage V.sub.RF is shown on the ordinate, and when an integrated voltage becomes equal thereto, the second blind of the shutter is started moving, thus closing the shutter.
More closely considering the curve Av shown in FIG. 5, it will be noted that it exhibits a significant time delay with respect to the ideal curve Fv, exhibiting a lower voltage than the latter at all times. It will be seen that time T.sub.4a corresponding to the point of intersection P.sub.2 between the curve Av and the reference voltage V.sub.RF is delayed with respect to time T.sub.3 corresponding to the point of intersection P.sub.F between the line Fv and the reference voltage V.sub.RF by an amount (T.sub.4a -T.sub.3), which represents an error in the exposure period. The curve Av has a break point P.sub.1, and thus is formed by a pair of rectilinear portions P.sub.0 -P.sub.1 and P.sub.1 -P.sub.2. Since the curve Av represents an integral of the photocurrent shown by the curve I.sub.A, as the first blind begins to open and until the time it moves part the center of the film (time Tc), the photoelectric transducer element receives light reflected from the surface of the first blind, and the resulting reflection is small, producing the photocurrent Is shown in FIG. 4. Accordingly, the integrated voltage increases in a rectilinear manner but at a lower rate, as indicated by line P.sub.0 -P.sub.1. When the first blind passes the center of the picture frame at time Tc, the transducer element receives light reflected from the film surface, producing the photocurrent I.sub.F. Hence the integrated voltage runs parallel to the line Fv, as indicated by line P.sub.1 -P.sub.2.
On the other hand, with respect to the curve Bv, the integrated voltage remains the same until point P.sub.3 which corresponds to time T.sub.1, but as the film surface begins to be exposed at time T.sub.1, the amount of light reflected increases gradually, and after point P.sub.4 corresponding to time T.sub.2 when the picture frame is fully open, the transducer element receives only light reflected from the film surface. Accordingly, the integrated voltage runs parallel to the line Fv. The curve Bv intersects with the level of the reference voltage V.sub.RF at point P.sub.5 corresponding to time T.sub.4b, which is delayed with respect to time T.sub.3 corresponding to the point of intersection P.sub.F between the line Fv and the reference voltage V.sub.RF, by an amount (T.sub.4b -T.sub.3), which also represents an error in the exposure period. A dotted line curve A.sub.R corresponds to the curve I.sub.AR shown in FIG. 4 and is offset from the curve Av in the region adjacent to the break point P.sub.1, in the same manner as the curve A.sub.R is offset from the ideal curve I.sub.A.
Thus it will be noted that the amount of light reflected from the film surface and the first blind surface, formed by a black cloth, depends on the reflectivity of each surface. The photocurrent produced by the reflection from the film surface having an increased reflectivity is greater in magnitude than the photocurrent which result from the reflection from the black cloth surface of the first blind having a reduced reflectivity. Hence, the integrated voltage, representing an integral of such photocurrent, results as shown in FIG. 5.
It will be appreciated that an integrated voltage of a greater magnitude can be obtained from the reduced photocurrent which results from the reflection from the black cloth surface of the first blind if an integrating capacitor C.sub.1 of a reduced capacitance is used. FIG. 6 graphically shows an integrated voltage which is obtained in this manner and which is shown by a curve E.sub.v which is offset from the ideal line Fv. The curve E.sub.v consists of a pair of rectilinear lines which are joined together at a break point P.sub.6 corresponding to time Tc. The line portion PG-P.sub.6 of the curve E.sub.v is obtained by using an integrating capacitor of a reduced capacitance and has the same inclination as the ideal curve Fv. The line portion beyond the break point P.sub.6 deflects upwardly, in the same manner as the curves Av, Bv deflect upwardly after time Tc in FIG. 5 since then the amount of light being reflected increases to increase the photocurrent after time Tc. Hence, the upward deflection of the curve E.sub.v after point P.sub.6 represents a deviation from the ideal line Fv.
Means is provided in the prior art to connect a correcting capacitor in parallel with the integrating capacitor C.sub.1 at time Tc in order to bring the curve E.sub.v of FIG. 6 into coincidence with the ideal line Fv. FIG. 7 shows a photometric circuit which is provided with such correction means. Specifically, the circuit arrangement is similar to that shown in FIG. 1 except that a correcting capacitor C.sub.2 and a switch SWc is added thereto. The correcting capacitor C.sub.2 may be connected in parallel with the integrating capacitor C.sub.1 through the switch SWc. The switch SWc is closed at time Tc corresponding to the described break point as the first blind runs. When the switch SWc connects the correcting capacitor C.sub.2 in parallel relationship with the integrating capacitor C.sub.1 at the break point P.sub.6 of FIG. 6, a deviation of the integrating voltage can be prevented, and the curve E.sub.v can be maintained close to the ideal line Fv.
However, with the photometric circuit shown in FIG. 7, the correcting capacitor C.sub.2 will be connected through the switch SWc while it is uncharged, so that the charge on the integrating capacitor C.sub.1 will be momentarily discharged through the switch SWc to the empty correcting capacitor C.sub.2. This results in an instantaneous reduction in the integrated voltge across the capacitor C.sub.1 as shown in FIG. 8. FIGS. 8(a), (b) and (c) show three different curves Ga, Gb an Gc for the integrated voltage. The reference voltage V.sub.RF is shown on the ordinate. For the curve Ga shown in FIG. 8(a), this curve will intersect with the reference voltage V.sub.RF at two points Pa.sub.0, Pa.sub.1. In this instance, the control circuit CT (see FIG. 7) will respond to the point Pa.sub.0 when determining an exposure period, and hence no problem is caused. However, for the curve Gb shown in FIG. 8(b), the point of intersection Pb.sub.0 between the curve Gb and the reference voltage V.sub.RF is unstable when it occurs in time coincidence with time Tc. Any slight change in the amount of light being reflected (a decrease is shown) causes a corresponding change in the integrated voltage. In this instance, the reference voltage V.sub.RF will not be reached, and only the other point of intersection Pb.sub.1 will be effective. In other words, the point in time when the exposure is to be terminated shifts from point Pb.sub.0 to point Pb.sub.1. This means that if the correcting capacitor C.sub.2 is connected into the circuit at a point adjacent to the termination of the proper exposure period, the resulting exposure may be greatly influenced. In particular, when the connection takes place at a point close to the fully open position of the shutter around 1/60 second, the influence becomes particularly significant. For the curve Gc shown in FIG. 8(c), the intersection with the reference voltage V.sub.RF takes place at a single point Pc, and hence poses no problem.